Add floating_toggle_hide command to show/hide floating windows.

Patch status: needinfo

Patch by Fabián Ezequiel Gallina

Long description:

The toggle is controlled by a new floating_hidden attribute in the Con
struct. This new attribute is used only for workspaces and its
existence is also reflected in IPC calls.
Issuing the floating_toggle_hide command does not affect scratchpad
windows and its effect is always local to a single workspace.
fixes #807